1. Field of the Invention
The present invention pertains in general to adaptive optics systems for correcting and preconditioning laser beams, and in particular, in one of the preferred embodiments, to the integration of wavefront sensor and correction functions into one integrated device.
2. Background Information
Since the invention of the laser in the 1950s, the optics industry has succeeded in vastly improving the power and utility of coherent light sources. The amount of radiant energy which can be transmitted over great distances with a minimum of scattering and diffraction losses has increased dramatically. In addition, a great number of applications have been developed for exploiting the spectral purity and spatial coherence of the laser beam. Communications, data transfer, and the projection and processing of images have come to depend upon the unique properties of the coherent laser wavefront. These properties must be preserved if the powerful and beneficial qualities of laser radiation are to be fully utilized. Except in free space, a laser beam travels through a material medium. When a laser beam propagates through glass, salt, quartz lens arrangements, optical fibers, or the atmosphere, the wavefront quality of the laser beam is often degraded. Waves with good spatial quality become aberrated when they traverse such inhomogeneous media; for example, a plane wave might emerge with a randomly perturbed wavefront. The enhanced diffraction which is associated with such aberrated waves significantly reduces the ability to focus the beam to a high-quality beamspot or to efficiently transmit a communications signal to a remote receiver.
Another problem occurs when such laser wavefronts are transmitting large amounts of energy. Some portion of that energy is absorbed when it passes through a given optics system of lenses, mirrors, and other optical devices, or travels through the atmosphere. When a material absorbs energy, it gets heated and typically the index of refraction of the material changes. This change in index varies across a given beam profile, since the intensity of the beam and consequently the amount of heat absorbed at any particular point in the material vary as a function of location within the beam. Differences in index cause diffraction and/or refraction of a laser beam which passes through material. The consequent spreading of the incident laser beam due to the laser-induced index differential is termed "thermal blooming". Thermal blooming can seriously degrade performance of a laser system designed to transmit an appreciable amount of energy over a long distance. Due to thermal blooming, the transmitted beam will have a phase distribution which is so randomized across its wavefront by the time it reaches the receiving site, that even a refocusing of the beam cannot eliminate the destructive interference which results within the beam. As a result of the destructive interference, the laser beam will deliver only a small fraction of the original transmitted energy to the receiving site.
Adaptive optics systems have been developed to reduce the deleterious effects of atmospheric turbulence, thermal blooming, and irregularities within the optical train. These systems combine wavefront sensing and wavefront correction within a closed feedback loop in order to correct wavefront errors in any transmitted laser beam. A typical laser beam direction system might work as follows. A laser beam from a laser source is directed via an atmospheric path to a target or receiving site. Because of optical path aberrations, such as turbulence and thermal blooming, only a portion of the radiation will reach the receiving site. In some systems, a laser reference signal is transmitted through the atmosphere and is used as a probe wave to sample the atmospheric aberrations. Alternatively, corner reflectors or target glints can be used to reflect incident laser radiation to achieve the same result. The wavefront of the reflected or reference signal suffers phase aberrations as it travels through the atmosphere. If these phase aberrations are then sensed, and the laser beam of interest is pre-distorted before transmission through the atmosphere, to exhibit this identical aberration, the aberrations will be almost exactly reversed or compensated-for as the laser beam travels through the atmosphere, and the laser beam will arrive at the target or receiving site unaberrated. Therefore, there will be practically no loss of beam energy due to atmospheric turbulence during transmission. The full amount of beam energy will therefore arrive within a diffraction-limited spot at the receiving site.
A variety of apparatus and methods have been developed or proposed to achieve this type of beam correction. Of these, the deformable mirror is perhaps the most popular and most easily understood. The deformable mirror is composed of a thin flexible glass, metallic sheet, or metallized membrane behind which is an array of piezoelectric or solenoid actuators. These actuators are push-pull devices which deform the mirror surface from its normal planar state. The reference signal traveling towards the laser source, strikes this deformable mirror, and is reflected through the adaptive optics system. The phase distribution of the aberrated wavefront is measured by any one of a number of standard techniques which are well known to those persons skilled in the art. This phase distribution is converted into electronic signals by a wavefront error sensor, and these electronic signals govern the voltages to be applied to each actuator. Then the actuators proportionately deform portions of the mirror. The system continuously adjusts front contour of the deformable mirror until the reference signal reflected by it has an unaberrated wavefront. After this happens, when the laser signal of interest is reflected by this deformed mirror, the reflected signal will be the time-reversed phase conjugate of the reference signal and will arrive at the receiving or target site almost completely unaberrated.
The deformable mirror has many inherent problems and drawbacks, some of which are discussed below. The use of discrete, bulky electrical actuators limits the spatial frequency response of the mirror. Moreover, a deformable mirror simply cannot correct errors finer than the spacing of the push-pull actuator elements. In addition, these actuators typically require several thousand volts for operation and are subject to arc-overs and permanent breakdowns. Another drawback is that the temporal frequency response of the adaptive system is limited by the impedance of the actuation device and the mass of the mirror. Each detector-actuator feed-back loop requires discrete electronic processing systems. The thin front surface of the mirror continuously experiences flexures. These flexures and other variations in the system materials, including the bonding material, contribute to drift and creep problems which adversely affect performance.
In an attempt to improve upon the performance of the deformable mirror, other phase-conjugation approaches have been attempted. Nonlinear optical media, using stimulated Brillouin, Raman scattering or degenerate four-wave mixing, have been used to provide a time-reversed wave as an output in some applications. In these methods, the medium is pumped by one or more local laser beams and the electric field of the aberrated reference signal, upon entering the phase-conjugation cell, causes index variations within the nonlinear medium in exact correspondence to the interference pattern between the remote reference signal and the local pump beams. When an incident laser beam is reflected by this index grating, the phase of the reflected beam has a field which is nearly the conjugate of that of the incident beam, and the reflected beam reaches the receiving site with minimal aberrations.
Although this phase-conjugation method greatly improves the spatial resolution of the system (the "actuator spacing" is now limited by the size of the atoms or molecules comprising the nonlinear medium), it also suffers from serious problems. Since the input sensitivity is very low, a fairly large return signal is necessary in order to set up the proper index grating structure within the phase-conjugation cell. Power requirements make this method unsuitable for low-power optical communication and laser power transmission when the reference signal is very weak, as is typically the case. Thus, the low sensitivity and, in some instances, the low efficiency of conversion make phase conjugation an interesting but often impractical method for long-range adaptive optics applications.
None of the methods or devices described above provide an efficient and comprehensive solution to all of the problems of correcting the phase of an aberrated light wave. None of these devices provide a versatile, highly sensitive, compact, and simple apparatus for adaptively correcting coherent light wavefronts. Therefore, there is a need, felt by the optical community for over two decades, for a practical and reliable method and apparatus for precisely correcting the phase of a given light wave using adaptive optics. Utilization of such a device within a laser beam direction system would enable significant improvement in performance of the system in correction of atmosphere-created wavefront aberrations. Such an invention would be ideally suited for operation in cooperation with a wide variety of adaptive optics systems and would enhance the performance of any optical apparatus requiring high-resolution wavefront correction.
An all-optical phase compensation system was described by Cardinal Warde et al in "High Resolution Adaptive Phase Compensation for Low-Visibility Optical Communication", Proc. IEEE, vol. 68, pp. 539-545, (1980). In this article, the authors examine the system requirements dictated by the coherence parameters of low-visibility wavefronts and describe a class of all-optical systems being investigated for high resolution, real time, wavefront phase compensation. In the all-optical systems described therein, the optical output from an interferometric phase sensor is used to drive an optically addressed spatial phase modulator in the path of the received beam. Such systems are described therein as automatically performing phase compensation or phase conjugation without discrete electronic processing. A configuration illustrated and discussed in detail therein features an interferometer of the homodyne type and a microchannel spatial light modulator (MSLM). The MSLM is described by Warde et al as consisting essentially of a photocathode, a microchannel array plate, and an electro-optic plate with a high resistivity dielectric mirror on one side and a transparent conducting electrode on the other.
The present invention, as discussed in detail later, in a preferred embodiment, uses a modified liquid crystal light valve (LCLV) with a parallel-aligned bire-fringent liquid crystal layer to provide an integrated adaptive optics apparatus. The present inventive apparatus is capable of compensating for wavefront aberration with high spatial resolution, and capable of operating with error signals encoded on optical beams of low intensity, with minimal total operating power and low operating voltages, while being able to handle multiple discrete-channels, typically on the order of thousands, and sometimes as high as one million. Moreover, the present inventive modified LCLV integrated apparatus is operable in several different operational modes, namely wave conjugation, wave scrubbing and wave replication modes, thereby being useful for several different adaptive optics applications.